Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2005 was an estimated 12.2 billion gallons. The global market for the fuel ethanol industry has also been predicted to continue to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends, the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, and the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major, free, energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually.
Catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. Micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as a sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source, all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al., Archives of Microbiology 161, pp 345-351 (1994)).
Although processes for the fermentation of substrates containing CO and H2 by microorganisms are known, the potential for scaling and integrating these processes into an industrial context has barely been explored. Petrochemical plants and oil refineries produce large quantities of CO as by-products and the potential exists to use this “waste” gas to produce valuable products. Additionally, a significant proportion of the waste gases are currently sent to flare (burned), or alternatively used as a source of fuel, both of which produce the undesirable greenhouse gas CO2. Accordingly, there exists the potential to make improvements to industrial processes by exploiting the waste gases and energy produced thereby for use in fermentation to produce desirable products while simultaneously reducing gaseous carbon emissions from industrial plants.
Hydrogen is predicted to become a major feedstock for use in hydrogen fuel cells which are being developed for use in technology ranging from cars to consumer electronics. Further, it may be used as a combustible fuel. Hydrogen is also required in refineries for a large number of hydrotreating and hydrocracking processes, to remove sulphur, nitrogen and other impurities from hydrotreater feed and to hydrocrack heavier gas oils to distillates. As hydrogen production is capital intensive, it is desirable to develop methods that increase hydrogen production and recovery efficiency, especially from low-purity streams. In the absence of hydrogen recovery, such streams end up in fuel gas or sent to flare and the high-value hydrogen component is effectively wasted.
It is an object of the present invention to provide a process that overcomes or ameliorates at least one of the disadvantages of the prior art, or at least to provide the public with a useful choice.